3-D nonvolatile memory device and method of manufacturing the same

ABSTRACT

A three-dimensional (3-D) nonvolatile memory device includes channel layers protruding perpendicular to a surface of a substrate, interlayer insulating layers and conductive layer patterns alternately formed to surround each of the channel layers, a slit formed between the channel layers, the slit penetrating the interlayer insulating layers and the conductive layer patterns, and an etch-stop layer formed on the surface of the substrate at the bottom of the slit.

CROSS-REFERENCE TO RELATED APPLICATION

Priority to Korean patent application number 10-2011-0139988 filed onDec. 22, 2011, the entire disclosure of which is incorporated byreference herein, is claimed.

BACKGROUND

Embodiments of this disclosure relate to a semiconductor device and amethod of manufacturing the same and, more particularly, to athree-dimensional (3-D) nonvolatile memory device and a method ofmanufacturing the same.

Technology for memory devices is developing toward a high degree ofintegration. In order to improve the degree of integration of memorydevices, schemes for reducing the size of memory cells arranged in a 2-Dway have been developed. As the size of the memory cells of the 2-Dmemory device is reduced, a multi-level cell (MLC) operation is notnormally performed due to increased interference and disturbance. Inorder to overcome the limit of the 2-D memory device, a 3-D structuredmemory device for improving the degree of integration by arrangingmemory cells over a substrate in a 3-D way has been proposed. The 3-Dstructured memory device can improve the degree of integration, ascompared with the case where memory cells are arranged in a 2-D waybecause the area of the substrate can be efficiently used.

The memory cells of the 3-D memory device include a plurality ofconductive layers and a plurality of interlayer insulating layersalternately stacked and vertical channel layers configured to penetratethe conductive layers and the interlayer insulating layers. A variety oftechniques are recently being proposed in order to improve reliabilityof the 3-D memory device.

BRIEF SUMMARY

An exemplary embodiment of this disclosure a 3-D nonvolatile memorydevice and a method of manufacturing the same.

In an aspect of this disclosure, a 3-D nonvolatile memory deviceincludes: channel layers protruding perpendicular to a surface of asubstrate; interlayer insulating layers and conductive layer patternsalternately formed to surround each of the channel layers; a slit formedbetween the channel layers, the slit penetrating the interlayerinsulating layers and the conductive layer patterns; and an etch-stoplayer formed on the surface of the substrate at a bottom of the slit.

In another aspect of this disclosure, a method of manufacturing a 3-Dnonvolatile memory device includes: forming a stack structure byalternately forming first material layers and second material layersover a substrate; forming channel layers extending perpendicular to asurface of the substrate, the channel layers penetrating the first andthe second material layers; forming a slit, exposing a portion of thesurface of the substrate, by etching the first material layers and thesecond material layers between the channel layers; and forming anetch-stop layer on the portion of the surface of the substrate exposedthrough the slit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a 3-D nonvolatile memory deviceaccording to an exemplary embodiment of this disclosure;

FIGS. 2A to 2H are cross-sectional views showing a method ofmanufacturing the 3-D nonvolatile memory device according to anexemplary embodiment of this disclosure;

FIG. 3 shows a cross-sectional view of a 3-D nonvolatile memory deviceaccording to another embodiment of this disclosure; and

FIG. 4 is a schematic block diagram of a memory system according to anexemplary embodiment of this disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, some exemplary embodiments of the present disclosure willbe described in detail with reference to the accompanying drawings. Thefigures are provided to allow those having ordinary skill in the art tounderstand the scope of the embodiments of the disclosure.

FIG. 1 is a perspective view showing a 3-D nonvolatile memory deviceaccording to an exemplary embodiment of this disclosure.

The 3-D nonvolatile memory device, according to an exemplary embodiment,includes a plurality of vertical channel layers (for example, CH1 andCH2), upwardly protruding from a substrate. A pipe gate PG, a pluralityof interlayer insulating layers 113A-113E, and a plurality of conductivelayer patterns 133 a, are alternately stacked around the verticalchannel layers CH1 and CH2. The conductive layer patterns 133 a may beused as word lines WL.

The pipe gate PG may be a semiconductor substrate or may be formed of aconductive layer formed on the semiconductor substrate. The pipe gate PGincludes a first conductive layer 111A and a second conductive layer111B formed on the first conductive layer 111A. The second conductivelayer 111B is a capping layer. The plurality of vertical channel layersCH1 and CH2 penetrates the second conductive layer 111B. An etch-stoplayer 127 is formed on a surface of the second conductive layer 111Bbetween the vertical channel layers CH1 and CH2. The etch-stop layer 127is made of a material having a great etch selectivity for the first andthe second conductive layers 111A and 111B or the conductive layerpatterns 133 a. Therefore, the etch-stop layer 127 may be etched at amuch slower rate (if at all) than the materials that form the first andthe second conductive layers 111A and 111B or the conductive layerpatterns 133 a. For example, the etch-stop layer 127 may be an oxidelayer formed by oxidizing a surface of the second conductive layer 111B.

The conductive layer patterns 133 a and the interlayer insulating layers113A-113E are separated by a slit 125 that is formed between thevertical channel layers CH1 and CH2. The slit 125 penetrates theconductive layer patterns 133 a and the interlayer insulating layers113A-113E. The slit 125 is filled with an insulating layer 135.

The etch-stop layer 127 is formed on a surface of the second conductivelayer 111B at a bottom of the slit 125. As will be described later, theconductive layer patterns 133 a are formed from a conductive layer thatis also formed in the slit 125 and trenches T. Accordingly, theconductive layer, formed within the slit 125, will be removed so thatthe conductive layer patterns 133 a remain only within the trenches T.The etch-stop layer 127 functions to protect the substrate, includingthe pipe gate PG, during the process of removing the conductive layerformed within the slit 125. In particular, the etch-stop layer 127 canprotect the second conductive layer 111B when performing the process ofremoving the conductive layer formed within the slit 125.

If the conductive layer patterns 133 a are made of metal materials oflow resistance (for example, tungsten), in order to lower resistance,the conductive layer patterns 133 a may be surrounded by a barrier metallayer patterns 131 a. The barrier metal layer patterns 131 a may be madeof, for example, titanium nitride (TiN).

A plurality of pipe trenches T-PG is formed in the first conductivelayer 111A, and each of the plurality of pipe trenches T-PG is filledwith a pipe channel layer CH_P. The pipe channel layer CH_P couples apair of the vertical channel layers CH1 and CH2. As a result, U-shapedchannel layers CH, each including the first and the second verticalchannel layers CH1 and CH2 and the pipe channel layer CH_P, are formed.

An outer surface of the U-shaped channel layer CH is surrounded by amemory stack layer 121. The channel layer CH may be formed by fillingthe memory stack layer 121, with a semiconductor layer as shown inFIG. 1. Alternatively, the channel layer may also include an insulatinglayer formed in a center portion of the semiconductor layer The memorystack layer 121, which surrounds the outer wall of the channel layer CH,may include a tunnel insulating layer, a charge trap layer, configuredto surround the tunnel insulating layer, and a charge blocking layerconfigured to surround the charge trap layer.

The sidewalls and bottom of the pipe channel layer CH_P are surroundedby the first conductive layer 111A, and the top of the pipe channellayer CH_P is covered with the second conductive layer 111B. An electricfield applied to the pipe channel layer CH_P can be enhanced through thesecond conductive layer 111B.

Memory cell transistors are formed at the intersections of the wordlines WL and the vertical channel layers CH1 and CH2, and a pipetransistor is formed at the intersection of the pipe gate PG and thepipe channel layer CH_P. For example, in an exemplary embodiment thememory cell transistors may be stacked along the vertical channel layersCH1 and CH2 in a 3-D array. Two columns of the memory cell transistorsare coupled by the pipe transistor, thus forming a U-shaped memorystring.

FIGS. 2A to 2H are cross-sectional views showing a method ofmanufacturing the 3-D nonvolatile memory device according to anexemplary embodiment.

Referring to FIG. 2A, the substrate includes the pipe gate layers PG,which are each filled with a sacrificial layer 112. The pipe gate layersPG, which are each filled with the sacrificial layer 112, each includethe first conductive layer 111A and the second conductive layer 111B,which is a capping layer. The pipe gate layers PG may be formed byforming pipe trenches T-PG within the first conductive layer 111A,filling each of the pipe trenches T-PG with the sacrificial layer 112,and stacking the second conductive layer 111B, which is the cappinglayer, on the first conductive layer 111A and the sacrificial layer 112.The first conductive layer 111A may be a semiconductor substrate or aconductive layer formed on the semiconductor substrate. The secondconductive layer 111B may be part of the semiconductor substrate or maybe a conductive layer, such as a polysilicon layer.

A stack structure is formed by alternately stacking a plurality of firstmaterial layers 113A-113E and a plurality of second material layers115A-115D over the substrate, including the pipe gate layer PG.

The first material layers 113A-113E are used to form interlayerinsulating layers that will insulate and separate conductive layerpatterns (to be formed later) from one another. The first materiallayers 113A-113E may be formed of oxide layers. The second materiallayers 115A-115D are used to form sacrificial layers, from which theconductive layer patterns will be formed. The second material layers115A-115D may be made of materials (e.g., nitride layers) having a greatetch selectivity for the first material layers 115A-115E. The number ofsecond material layers 115A-115D may be determined by the number ofmemory cells to be stacked.

In some embodiments, the first material layers 113A-113E may be formedof conductive layers that are sacrificial layers, and the secondmaterial layers 115A-115D, which form the conductive layer patterns, maybe formed of conductive layers that have a great etch selectivity forthe sacrificial conductive layers. For example, the first materiallayers 113A-113E may be formed of undoped polysilicon, and the secondmaterial layers 115A-115D may be formed of doped polysilicon.

For the sake of example, it is hereinafter assumed that the firstmaterial layers 113A-113E are interlayer insulating layers and that thesecond material layers 115A-115D are formed of insulating layers.

Referring to FIG. 2B, a plurality of holes H that expose the sacrificiallayer 112 is formed by etching the first and the second material layers113A-113E and 115A-115D. The holes H are formed to penetrate the firstand the second material layers 113A-113E and 115A-115D. If the secondconductive layer 111B is formed on the sacrificial layer 112, then thesecond conductive layer 111B will be further etched, in order to formthe plurality of holes H. Thus, the plurality of holes H furtherpenetrates the second conductive layer 111B. Here, a pair of the holes His coupled to each of the pipe trenches T-PG in the pipe gate layers PG.

If the sacrificial layer 112 does not have a great etch selectivity forthe first and the second material layers 113A-113E and 115A-115D, apassivation layer (not shown), having a great etch selectivity for thefirst and the second material layers 113A-113E and 115A-115D, may beformed on the sidewalls of the holes H, after forming the plurality ofholes H. If the sacrificial layer 112 has a great etch selectivity forthe first and the second material layers 113A-113E and 115A-115D, thenthe process of forming the passivation layer may be omitted.

Referring to FIG. 2C, the sacrificial layer 112 exposed through theholes H is removed. If the passivation layer has been formed on thesidewalk of the holes H in the previous process, a process of removingthe passivation layer may be further performed after removing thesacrificial layer 112.

The memory stack layer 121 is formed on a surface of the pipe trenchesT-PG in the pipe gate layers PG and on a surface of the holes H. Thememory stack layer 121 may include a charge blocking layer, a chargetrap layer formed on the charge blocking layer, and a tunnel insulatinglayer formed on the charge trap layer. The charge trap layer may beformed of a nitride layer capable of trapping charges.

A semiconductor layer is formed over the memory stack layer 121, therebyforming a channel layer CH, which includes the vertical channel layersCH1 and CH2, disposed within the holes H, and the pipe channel layerCH_P, formed in the pipe trenches T-PG in the pipe gate layers PG. Thevertical channel layers CH1 and CH2 include a first vertical channellayer CH1 coupled to one end of the pipe channel layer CH_P and a secondvertical channel layer CH1 coupled to the other end of the pipe channellayer CH_P. The channel layer CH may completely fill the hole H and thepipe trenches T-PG in the pipe gate layers PG. Alternatively, thechannel layer CH may have a hollow center, so as to have a tube shape.If the channel layer CH is formed in a tube shape, a process of fillingthe tube with an insulating layer (not shown) may be further performed.

The memory stack layer 121 and the channel layer CH may be formed on thefirst and the second material layers 113A-113E and 115A-115D outside ofthe holes H and the pipe trenches T-PG. In this case, after forming thememory stack layer 121 and the channel layer CH, the memory stack layer121 and the channel layer CH outside of the holes H and the pipetrenches T-PG are polished to expose first and the second materiallayers 113A-113E and 115A-115D. As a result, the memory stack layer 121and the channel layer CH remain only within the holes H and the pipetrenched T-PG.

Referring to FIG. 2D, slits 125 are formed by etching the first and thesecond material layers 113A-113E and 115A-115D. The slits 125 are formedbetween the vertical channel layers CH1 and CH2. Thus, sidewalls of thefirst and the second material layers 113A-113E and 115A-115D are exposedthrough the slit 125, and the first and the second material layers113A-113E and 115A-115D are split by the slit 125. Furthermore, asurface of the second conductive layer 11113 is exposed.

Referring to FIG. 2E, an etch-stop layer 127 is formed on the surface ofthe second conductive layer 111B exposed through the slit 125. Theetch-stop layer 127 has a great etch selectivity for the second materiallayers 115A-115E and a third conductive layer, to be formed later.

In order to form the etch-stop layer 127, an oxide layer may be formedby oxidizing the exposed surface of the second conductive layer 111Busing O₂ gas at a temperature of about 950° C. using, for example, arapid thermal oxidation (RTO) method. In some embodiments, the oxidelayer may be formed by oxidizing the exposed surface of the secondconductive layer 111B using H₂, O₂, N₂, or Ar gas at a temperature ofabout 400° C. to about 600° C., using a plasma method.

Referring to FIG. 2F, the second material layers 115A-115D are removedby an etch process. Thus, trenches T are formed between adjacent pairsof the first material layers 113A-113E. The second material layers115A-115D are made of materials having a great etch selectivity for theetch-stop layer 127 and of the first material layers 113A-113E. Thus,only the second material layers 115A-115D are etched.

Referring to FIG. 2G, a third conductive layer 133 is formed over theentire structure so that the trenches T are filled with the thirdconductive layer 133. When the third conductive layer 133 is formed of amaterial including metal materials of low resistance, then a barriermetal layer 131 may be formed before forming the third conductive layer133. The barrier metal layer 131 is formed along a surface of the entirestructure, in which the trenches T are formed. The third conductivelayer 133 is formed on the barrier metal layer 131 and fully fills thetrenches T.

The barrier metal layer 131 may be formed by depositing TiN using, forexample, an atomic layer deposition (ALD) method. Accordingly, thethickness of the barrier metal layer 131, formed on the etch-stop layer127 at the bottom of the slit 125, is substantially the same as thethickness of the barrier metal layer 131 formed on a surface of thetrenches T.

Referring to FIG. 2H, the barrier metal layer 131 and the thirdconductive layer 133, within the slit 125 and at the top surface of theupper most first material layer 113E, are etched so that the barriermetal layers 131 and conductive layer patterns 133 a remain within thetrenches T. Thus, the barrier metal layer patterns 131 a and theconductive layer patterns 133 a are split by the slit 125. Theconductive layer patterns 133 a may be used as word lines WL. Next, theslit 125 is filled within the insulating layer 135.

The process of etching the third conductive layer 133, is performedusing an etchant to selectively etch the third conductive layer 133.Etching may be performed up to the barrier metal layer 131 at the bottomof the slit 125. In accordance with one embodiment of invention, thesecond conductive layer 111B is protected by the etch-stop layer 127,during the etch process of the third conductive layer 133, because theetch-stop layer 127 has a great etch selectivity for the thirdconductive layer 133. Accordingly, damage to the substrate, includingthe second conductive layer 111B, during the etch process of the thirdconductive layer 133 can be reduced or prevented.

In an alternative embodiment (not shown), if the first material layers113A-113E are formed of sacrificial conductive layers and the secondmaterial layers 115A-115D are formed of third conductive layers, havinga great etch selectivity for the sacrificial conductive layers, thefollowing process, which is different than the process illustrated inFIGS. 2F to 2H, may be performed.

First, the first material layers 113A-113E are removed by an etchprocess, so that the trenches are formed between the second materiallayers 115A-115D of adjacent layers. The first material layers 113A-113Eare made of materials having a great etch selectivity for the etch-stoplayer 127 and the second material layers 115A-115D. Thus, only the firstmaterial layers 113A-113E may be selectively etched. Furthermore, duringthe etch process of the first material layers 113A-113E, which areformed of the sacrificial conductive layers, the substrate, includingthe second conductive layer 111B, is protected by the etch-stop layer127.

Next, interlayer insulating layers are formed by filling the regionsfrom which the first material layers 113A-113E have been removed with aninsulating material. Here, the insulating layer 135 may also be formedwithin the slit 125.

FIG. 3 shows a cross-sectional view of a 3-D nonvolatile memory deviceaccording to another embodiment of this disclosure.

The 3-D nonvolatile memory device, according to another exemplaryembodiment, includes a plurality of vertical channel layers CH, formedin holes H and upwardly protruding from a substrate 211, a plurality ofinterlayer insulating layers 213A-213E, a plurality of conductive layerpatterns 233 a, alternately stacked around the vertical channel layersCH, and a memory stack layer 221 configured to surround the outer wallsof the channel layers CH.

An impurity is implanted in a specific depth from a surface of thesubstrate 211, thus forming a source region 203 that functions as acommon source region or a common source line. An etch-stop layer 227 isformed on a surface of the source region 203 between the verticalchannel layers CH adjacent to each other. The etch-stop layer 227includes an oxide layer formed by oxidizing the surface of the sourceregion 203 of the substrate 211 as described above with reference toFIG. 1.

A vertical channel layer CH may be formed by filling a hole H with asemiconductor material, so that the vertical channel layer CH completelyfills the hole H. Alternatively, the vertical channel layer CH may havea hollow center, so as to have a tube shape. If the vertical channellayer CH is formed in a tube shape, a process of filling the tube withan insulating layer (not shown) may be further performed. The memorystack layer 221, which surrounds the outer walls of the vertical channellayers CH, includes a tunnel insulating layer, a charge trap layerconfigured to surround the tunnel insulating layer, and a chargeblocking layer configured to surround the charge trap layer.

The interlayer insulating layers 213A-213E and the conductive layerpatterns 233 a are separated by a slit 225. The slit 225 is formedbetween the adjacent vertical channel layers CH and is configured topenetrate the interlayer insulating layers 213A-213E and the conductivelayer patterns 233 a. The slit 225 is filled with an insulating layer235. A lowermost layer of the conductive layer patterns 233 a may becomea first select line LSL, and an uppermost layer of the conductive layerpatterns 233 a may become a second select line USL. Furthermore, theconductive layer patterns between the first and the second select linesLSL and USL may become word lines WL.

The etch-stop layer 227 is formed on the surface of the source region203 at the bottom of the slit 225. The etch-stop layer 227 may functionas an etch-stop layer in a process of removing the unnecessary regionsof the conductive layers so that the conductive layer patterns 233 aremain within trenches T. That is, within regions in which the firstselect line LSL, the word lines WL, and the second select line USL willbe formed.

If the conductive layer patterns 233 a are made of metal materialshaving low resistance (for example, tungsten), in order to improveresistance of the first and the second select lines LSL and USL and theword lines WL, the first and the second select lines LSL and USL and theword lines WL may further include barrier metal layer patterns 231 athat surrounds the conductive layer patterns 233 a. The barrier metallayer patterns 231 a may be made of TiN.

A first select transistor is formed at the intersection of the firstselect line LSL and the vertical channel layer CH, and a second selecttransistor is formed at the intersection of the second select line USLand the vertical channel layer CH. Furthermore, memory cell transistorsare formed at the intersections of the word lines WL and the verticalchannel layers CH. In accordance with the structure, the memory celltransistors according to an exemplary embodiment of this disclosure arestacked along the vertical channel layers CH, arranged in a 3-D way, andcoupled in series between the first and the second select transistors,thus forming memory strings.

The 3-D nonvolatile memory device may be manufactured as follows.

First, the source region 203, which becomes the common source region orthe common source line, is formed by implanting an impurity into thesemiconductor substrate 211 or by forming a doped polysilicon layer,into which an impurity has been doped, on the semiconductor substrate211. A plurality of first material layers and a plurality of secondmaterial layers are alternately stacked over the source region 203, asdescribed above with reference to FIG. 2A.

A plurality of holes is formed by etching the first and the secondmaterial layers, as described above with reference to FIG. 2B. Thememory stack layer 221 is formed on a surface of the holes, as describedabove with reference to FIG. 2C. Slits 125, through which a surface ofthe source region 203 is exposed, are formed by etching the first andthe second material layers, as described above with reference to FIG.2D. The etch-stop layer 227 is formed on the surface of the sourceregion 203 exposed through the slit, as described above with referenceto FIG. 2E. The process of forming the etch-stop layer 227 is the sameas that described with reference to FIG. 2E.

Next, the trenches T are formed by removing the second material layersusing an etch process, as described above with reference to FIG. 2F. Thebarrier metal layer and the conductive layer are formed within thetrenches T, as described above with reference to FIGS. 2G and 2H. Next,the barrier metal layer patterns 231 a and the conductive layer patterns233 a split by the gate lines LSL, WL, and USL are formed by etching thebarrier metal layer and the conductive layer. Next, the slit is filledwith the insulating layer 235.

FIG. 4 is a schematic block diagram of a memory system according to anexemplary embodiment of this disclosure.

Referring to FIG. 4, the memory system 400 according to the exemplaryembodiment of this disclosure includes a memory device 420 and a memorycontroller 410.

The memory device 420 includes at least one of the 3-D nonvolatilememory devices shown in FIGS. 1 and 3. That is, the memory device 420includes the vertical channel layers upwardly protruded from thesubstrate, the interlayer insulating layers and the conductive layerpatterns, alternately stacked around the vertical channel layers, theslit placed between the vertical channel layers, the slit configured topenetrate the interlayer insulating layers and the conductive layerpatterns, and the etch-stop layer formed on the surface of the substrateat the bottom of the slit.

The memory controller 410 controls the exchange of data between a hostand the memory device 420. The memory controller 410 may include acentral processing unit (CPU) 412 for controlling the overall operationof the memory system 400. The memory controller 410 may include SRAM 411used as the operating memory of the CPU 412. The memory controller 410may further include a host interface (I/F) 413 and a memory I/F 415. Thehost I/F 413 may be equipped with a data exchange protocol between thememory system 400 and the host. The memory I/F 415 may couple the memorycontroller 410 and the memory device 420. The memory controller 410 mayfurther include an error correction code (ECC) block 414. The ECC block414 can detect errors in data read from the memory device 420 andcorrect the detected errors. Although not shown, the memory system 400may further include a ROM device for storing code data for interfacingwith the host. The memory system 400 may be used as a portable datastorage card. In some embodiments, the memory system 400 may be embodiedusing a solid state disk (SSD) that may replace the hard disk of acomputer system.

In accordance with this disclosure, the etch-stop layer is formed on asurface of the substrate between the vertical channel layers.Accordingly, after forming the slits, the substrate under the etch-stoplayer can be protected from a process of etching the conductive layers,formed over the substrate, while etching the unnecessary parts of theconductive layers.

What is claimed is:
 1. A method of manufacturing a three-dimensional (3-D) nonvolatile memory device, the method comprising: forming a stack structure by alternately forming first material layers and second material layers over a substrate; forming channel layers extending perpendicular to a surface of the substrate, the channel layers penetrating the first and the second material layers; forming a slit, exposing a portion of the surface of the substrate, by etching the first material layers and the second material layers between the channel layers; and forming an etch-stop layer on the portion of the surface of the substrate exposed through the slit by changing the portion of the surface of the substrate into an insulating material.
 2. The method of claim 1, where: the first material layers are interlayer insulating layers, and the second material layers are sacrificial layers having an etch selectivity that is different from an etch selectivity of the first material layers.
 3. The method of claim 1, further comprising: forming trenches, by removing the second material layers exposed by the slit, after forming the etch-stop layer; forming a conductive layer over an entire structure, in which the trenches are formed, so that the trenches are filled with the conductive layer; and forming a conductive pattern, in each of the trenches, by etching the conductive layer within the slit so that each of the conductive patterns is separate from one another.
 4. The method of claim 3, further comprising: forming a barrier metal layer on a surface of the entire structure in which the trenches are formed, before forming the conductive layer.
 5. The method of claim 3, wherein etching the conductive layer within the slit is performed using an etchant to selectively etch the conductive layer.
 6. The method of claim 1, further comprising: forming pipe trenches, in the substrate, by etching the substrate before forming the stack structure; and forming a sacrificial layer to fill the pipe trenches.
 7. The method of claim 6, further comprising: forming holes by etching the first material layers and second material layers before forming the channel layers so that the sacrificial layer is exposed; removing the sacrificial layer; and forming a pipe channel layer in each of the pipe trenches.
 8. The method of claim 7, wherein the substrate comprises a capping layer formed over the pipe channel layer.
 9. The method of claim 1, wherein forming the etch-stop layer comprises: forming an oxide layer by oxidizing the portion of the surface of the substrate exposed through the slit. 